1. Field of the Invention
The present invention relates generally to a booster-compressor for a turboprop aircraft engine and particularly concerns a turboprop booster driven by a propeller hub. The booster is provided with specially designed rotors and stators which maintain a substantially constant static pressure from the booster inlet to the booster outlet.
2. Description of Prior Developments
In order to increase the thrust developed by conventional gas turbine Jet aircraft engines, supercharged air can be driven into the inlet of the core engine, also known as the gas generator, with a booster type compression system. Such a system typically includes several rows of rotor blades and several rows of coacting stator vanes which raise the pressure of the air entering the core engine.
A need has recently been recognized for increasing the thrust produced by existing turboprop engines. A turboprop engine is generally defined as an engine which does not have a duct surrounding its propeller blades as contrasted with a turbofan engine which is typically defined as an engine which includes a duct surrounding its rotating fan blades. Because of the differences in operation and design between gas turbine-driven jet engines and turboprop engines, conventional booster designs suitable for use with gas turbine jet engines do not appear to offer significant improvements in the thrust developed by turboprop engines.
For example, initial studies associated with the development of the present invention applied conventional booster design technology to a turboprop engine. These studies investigated the possibility of driving a turboprop booster on the same hub or spinner used to drive the propeller blades. Unfortunately, the wheel speed of such a booster is constrained by the rotational speed of the propeller or fan and the diameter of the propeller spinner.
Because of these constraints, booster designs of the type used in gas turbine engines would require at least two stages of compression to produce a modest five percent increase in air pressure. Achieving this relatively small pressure boost with two or more additional conventional compression stages has not been viewed as an attractive solution to increasing the thrust of a turboprop engine.
When an existing turboprop engine is to be supercharged, a problem can arise in sizing the rotor blades and stator blades of the booster-compressor if conventional design techniques are applied. That is, the required cross-sectional area of the annulus which defines the flowpath of the supercharged air into the core engine can in some cases be quite small. This can result in the use of large numbers of relatively small, difficult to manufacture, rotor blades and stator vanes.
Accordingly, a need exists for a booster-compressor for supercharging inlet air into the core engine of a turboprop engine using a minimum number of booster stages and a minimum number of blades and vanes. Ideally, a single stage booster having a single row of rotor blades and a single row of stator vanes would produce a significant increase in the pressure of the air introduced into the core engine of a turboprop aircraft engine.
A further need exists for such a turboprop booster which can be designed with a relatively large annular flowpath so as to allow the use of conventional sized rotor blades and stator vanes. Such blade and vane sizing would reduce the number of blades and vanes required and facilitate their manufacture and assembly.
An additional need exists for a booster which provides a greater increase in the pressure of supercharged air entering the core engine of a turboprop engine than that presently available by applying conventional gas turbine engine booster designs to a turboprop engine.
An additional need exits for a booster which can produce a given increase in pressure using the fewest number of compression stages.